1. Field of the Invention
The present invention relates to apparatus for use in cementing any tubular member that is run or landed with a string of drill pipe.
2. Description of the Prior Art
Prior art liner cementing equipment includes a flapper or a float valve at the bottom of the liner. The flapper is conventionally held open by a breakable plastic tab which is actuated (i.e., broken) by a drop ball when the cementing operation is to begin. Since the drop ball which is used to activate the float valve must be small enough to pass through the drill string, the maximum float valve opening size for a 133/8" liner may be on the order of 2 to 3" in diameter. As the liner is lowered into the well bore, the fluid in front of the liner must be displaced to flow through the bottom opening in the float valve as well as around the outside annulus defined by the well bore and the liner.
Two problems exist with respect to conventional cementing apparatus. First, the flow resistance of two above-identified flow paths may be high and thus cause a pressure buildup below the liner. This pressure buildup can: (a) cause damage to the formation; (b) result in loss of expensive drilling fluid; and (c) result in the liner sticking against the side of the borehole, which means that the liner does not go to the bottom of the hole. Second, the small opening through the float valve may, and often does, become plugged with cuttings and contaminants due to its small size and the highly contaminated environment in which it is used.
U.S. Pat. No. 5,960,881, which is incorporated herein by reference, discloses a downhole surge pressure reduction system to reduce the pressure buildup while running in liners. The system is typically located immediately above the top of the liner. Nonetheless, any plugging of the float valve at the lower end of the liner can, and very well may, render the surge pressure reduction system of the '881 patent ineffective.
The method and apparatus according to the present invention overcomes the plugging problem which has heretofore existed.